Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows into a combustion chamber where the compressed working fluid mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases expand in the turbine to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Various design and operating parameters influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by fuel nozzles, possibly causing severe damage to the fuel nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, one or more late lean injectors or tubes may be circumferentially arranged around the combustion chamber downstream from the fuel nozzles. Fuel may be injected through fuel ports in the tubes, and a portion of the compressed working fluid exiting the compressor may flow through the tubes to mix with the fuel to produce a lean fuel-air mixture. The lean fuel-air mixture may then flow through the tubes into the combustion chamber to produce additional combustion that raises the combustion gas temperature and increases the thermodynamic efficiency of the combustor.
The late lean injectors have proven to be effective at increasing combustion gas temperatures without producing a corresponding increase in the production of NOX, and various efforts have been made to enhance the performance of the late lean injectors with varying success. For example, the number and/or size of the fuel ports have been increased to increase the amount of fuel that may be supplied through the late lean injectors. However, the additional and/or larger fuel ports reduce the penetration of the fuel inside the tubes and/or allow the injected fuel to coalesce inside the tubes which adversely impacts combustor emissions. Therefore, an improved system for supplying working fuel to the combustor would be useful.